Selectivity
Generally, the stable isotope-labeled analogue is a desirable
internal standard for quantitative analysis in GC–MS. We compared
isobutane with acetylene-d2; however, because acetylene-d2 possesses
fragment ion m/z 26, ca. 30% of the molecular ion (Fig. 1),
it was considered that acetylene-d2 is not suitable under these
conditions; therefore, we used isobutane as the I.S.
Under GC–MS conditions, acetylene and isobutane appeared at
2.91 min and 5.41 min, respectively. The characteristic mass spectrum
of acetylene indicating the molecular ion at m/z 26 was also
observed. The acetylene peak was clearly distinguishable from air
peaks (containing oxygen, nitrogen, argon and carbon dioxide) and
I.S. Acetylene and isobutane used as standard substances were not
contaminated in this analysis.